In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The wireless terminals can be mobile stations or user equipment units (UE) such as mobile telephones (“cellular” telephones) and laptops with wireless capability), e.g., mobile termination), and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called “NodeB” or “B node”. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions (particularly earlier versions) of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
Future wireless systems include long term evolution (LTE) and Worldwide Interoperability for Microwave Access (WiMAX). Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Thus, 3GPP LTE (Long Term Evolution) is the name given to a project within the Third Generation Partnership Project to improve the UMTS mobile phone standard to cope with future requirements. See, e.g., Dahlman et al, 3G Evolution; HSPA and LTE for Mobile Broadband, Academic Press Inc., U.S., 2007, and 3GPP TS 36.211, Physical Channels and Modulation.
WiMAX is a telecommunications technology aimed at providing wireless data over long distances in a variety of ways, from point-to-point links to full mobile cellular type access. See IEEE 802.16-2004 Standard for Local and metropolitan area networks Part 16: Air Interface for Fixed Broadband Wireless Access Systems
LTE and WiMAX utilize Multiple-Input Multiple-Output (MIMO) transmission schemes to increase the spectral efficiency. MIMO schemes assume that the transmitter and receiver are both equipped with multiple antennas, and that multiple modulated and precoded signals are transmitted on the same “time-frequency resource element”. See FIG. 6 and FIG. 7. FIG. 7 is arranged to have matrices in three rows and four columns. The first column of FIG. 7 is for antenna port 0; the second column of FIG. 7 is for antenna port 1; the third column of FIG. 7 is for antenna port 2; the fourth column of FIG. 7 is for antenna port 3. The top row of FIG. 7 is for one antenna port; the middle row of FIG. 7 is for two antenna ports; the bottom row of FIG. 7 is for four antenna ports. Each matrix of FIG. 7 also has rows and columns. The first seven columns of each matrix are for even-numbered slots; the last seven columns of each matrix are for odd-numbered slots.
In MIMO technology, mathematically the transmitted signal for a particular frequency/time resource element (k,l) can be expressed by Expression (1).x(k,l)=W(k)s(k,l)  Expression (1)In Expression (1), s is a vector with elements Si, i=1, . . . , Ns, and where Si is a modulated symbol and Ns is the number of transmitted layers; W(k) is the so-called precoding matrix of dimension Nlx×Ns, where Nlx is the number of transmitted antennas; x is a vector of transmitted signals, where xi, i=1, . . . , Nlx, is the signal transmitted from the ith transmit antenna. As used herein, “k” and “l” are the frequency and time indices, respectively, and each element in vectors x and s are given for a particular frequency/time.
The signal is transmitted over a channel which can be characterized by a channel matrix H, the channel matrix H being of dimension Nrs×Nlx, where Nr is the number of received antennas. The received signal vector is then an Nrx dimensional vector given by Expression (2).y=Hx+e=HWs+e  Expression (2)In Expression (2), e is a noise and interference vector, with covariance matrix Re. In the following text the indices (k,l) have been omitted to simplify notation.
Several known techniques exist for demodulating the signal s from the received data, e.g., from the received signal vector y. Among the demodulation techniques are the Minimum Mean Square Error (MMSE) technique and the Interference Rejection Combining (IRC) technique. The Interference Rejection Combining (IRC) technique typically uses an expression such as Expression (3).ŝt=λgiH(GGH−gigiH+Re)−1y  Expression (3)Expression (3) provides an estimate of the ith transmitted signal. In Expression (3), G=HW=[g1, . . . gs]; λ is a scaling factor that makes the estimator unbiased. In Expression (3) and elsewhere herein, the superscript H denotes a Hermitian transpose. The receiver requires knowledge of the channel H (which can be estimated using reference symbols), the precoder matrix W used for the transmitted signal (which is usually signalled to the UE), and the covariance matrix Re. The covariance matrix Re can, e.g., be estimated using, e.g., the residuals from the channel estimation.
A special case arises when W is a column vector, i.e., when Ns=1. In this case a Maximum Ratio Combining (MRC) receiver as depicted by Expression (3b) can be utilized.ŝt=λgiHRe−1y  Expression (3b)
Multi-User MIMO (MU-MIMO) is an extension of MIMO where signals are transmitted to multiple users, such that a transmitter signal to a first user is depicted by Expression (4a) and a transmitter signal to a second user is depicted by Expression (4b).x1=W1s1  Expression (4a)x2=W2S2  Expression (4b)At the first receiver/user of a MU-MIMO system, the received signal is depicted by Expression (5).y=H(W1s1+W2s2)+e=HW1s1+HW2s2+e  Expression (5)For such first receiver/user, the desired signal is HW1s1, and accordingly the noise plus interference is provided by Expression (6). The noise plus interference has a covariance matrix described by Expression (7).ν=HW2s2+e  Expression (6)Rν=HW2W2HHH+Re  Expression (7)
For a MU-MIMO system, the receiver/user equipment unit (UE) can (as mentioned above) estimate the channel matrix H based on reference symbols transmitted from all transmit antennas. The reference symbols are typically transmitted on orthogonal resources, i.e. a resource element used for transmitting reference symbols from one antennas is not used by any other antenna, essentially in the manner illustrated by FIG. 2. As a consequence, the interference part of Expression (5) is not possible to estimate using reference symbols only, since these carry no information about the term HW2s2.
Thus, in the existing technology it is difficult to estimate the noise for a MU-MIMO transmission. One attempted prior art solution was to signal explicitly to a receiving user equipment unit (UE) which precoder matrix W2 is used for transmission to the other user equipment unit, so that the receiving user equipment unit could try to determine the noise caused by the other user equipment unit. However this signaling of precoder matrix for another UE costs too much in terms of, e.g., signalling overhead.
Another attempted prior art solution was to use the aforementioned Minimum Mean Square Error (MMSE) technique. In the Minimum Mean Square Error (MMSE) technique, the weights (Rν) are defined by Expression (8), which in turn is utilized by Expression (9) to determine symbol estimate ŝi.
                              R          v                =                              1                          N              RE                                ⁢                                    ∑                              k                ,                l                                      ⁢                                          y                ⁡                                  (                                      k                    ,                    l                                    )                                            ⁢                                                y                  ⁡                                      (                                          k                      ,                      l                                        )                                                  H                                                                        Expression        ⁢                                  ⁢                  (          8          )                                                              s            ^                    t                =                  λ          ⁢                                          ⁢                      g            t            H                    ⁢                      R            v                          -              1                                ⁢          y                                    Expression        ⁢                                  ⁢                  (          9          )                    
A receiver that uses the Minimum Mean Square Error (MMSE) technique performs rather well in presence of MU-MIMO interference, but in absence of such interference, performance degrades. So from a robustness point of view, Minimum Mean Square Error (MMSE) technique is not a practical solution, since it needs to know when interference is present or not.
What is needed, therefore, and an object and/or advantage of the present invention, are one or more of apparatus, systems, methods, and techniques which facilitate noise estimation for MU-MIMO transmissions.